Audio mixing is used for sound recording, audio editing, and sound systems to balance the relative volume, frequency, and dynamical content of a number of sound sources. Typically, these sound sources are the different musical instruments in a band or vocalists, the sections of an orchestra, announcers and journalists, crowd noises, and so on.
Sometimes audio mixing is done live by a sound engineer or recording engineer, for example at rock concerts and other musical performances where a public address system (PA) is used. Audio mixing may also be done in studios as part of multi-track recording in order to produce digital or analog audio recordings, or as part of an album, film, or television program. An audio mixing console, or mixing desk, or mixing board, has numerous rotating controls (potentiometers) and sliding controls (faders which are also potentiometers) that are used to manipulate the volume, the addition of effects such as reverb, and frequency content (equalization) of audio signals. On most consoles, all the controls that apply to a single channel of audio are arranged in a vertical column called a channel strip. Larger and more complex consoles such as those used in film and television production can contain hundreds of channel strips. Many consoles today, regardless of cost, have automation capabilities so the movement of their controls is performed automatically, not unlike a player piano.
Certain terms used herein will now be defined. RMS (root means square) is a level value based upon the energy that is contained in a given audio signal. Peak value describes the instantaneous maximum amplitude value within one period of the signal concerned. DAW (digital audio workstation) is a software environment used to record, edit and mix audio files. Crest factor is the peak/RMS ratio. Loudness Unit (LU) is a unit that considers the perceived loudness of an audio signal regarding duration and frequency weighting. Keyframes are level changes in an audio track, and wherein the slope of the change or the time required to transition from one level to another can be adjusted.